Cause:
If you leave the system at the Windows logon screen (user ID & password entry screen), neither PowerChute Business Edition nor PowerChute Network Shutdown can shutdown Windows during power failure or scheduled shutdown. It is a Windows' specification.

Under normal circumstance, you get the logon screen after CTRL+ALT+DEL and do not key in any data for about 30 seconds, the system will bring back the CTRL+ALT+DEL screen.

Solution:

Do not enable the "Disable CTRL+ALT+DEL requirement for logon" option found in the Security option of Local Security settings tools in Administrative tools. Do not leave your system with the logon screen.
If you are running Windows XP Professional x64 Edition and it is not joined to a domain, follow the steps below to enable the CTRL+ALT+DEL screen.
Start gpedit.msc from the command prompt.
Select Local Computer Policy > Computer Configuration > Administrative Template> Logon
Enable the "Always use classic logon" option